As an output apparatus for a computer, an image data output apparatus in which a laser beam printer or the like is used, and more particularly, a recording apparatus has been widely used. Since many advantages such as high image quality, low noise, and the like can be obtainable from the apparatus of the type described above, the field of the disk top publishing (to be abbreviated to "DTP") has been rapidly promoted.
Furthermore, due to the current tendencies of a high memory capacity, high speed processing, low cost, high grade function realized in the host computers or the controllers for the printers above, a variety of processings can be performed not only in the binary, that is, black and white printing but also in a dither method, density pattern method, or expressing of half tone images by means of pulse width modulation. In the current tendency above, a laser beam printer has been widely used as an alternative to a wire dot printer due to its high quality image recording performable at the time of recording a so-called text data in a word processing field. The major portion of the laser beam printers capable of printing text data has been conventionally arranged to display a record density of 240 dpi (dot/inch) or 300 dpi (dot/inch). The thus-arranged record density for the laser printers is a density of the level that is at least higher than that arranged for the wire dot printers, and these recording density is capable of realizing a sufficiently high grade image as the output in the usual word processing operations. As for the memory for the controller, thanks to the tendency of the relative cost reduction, the capacity of the level with which the recording density of the level described above can be secured.
For example, a laser beam printer capable of performing a printing with the printing density of 300 dpi comprises, as shown in FIG. 1:
a printer engine portion 151 capable of conducting a printing on a photosensitive drum on the basis of dot data; and
a printer controller 152 arranged to receive code data transmitted from an external host computer 154, to generate page information consisting of dot data on the basis of the thus-received code data, and to transmit dot data successively to the printer engine portion 151. The host computer 154 is arranged to be loaded with a program by a floppy disk 155 having an application software so that this application software is started so as to function as, for example, a word processor. A variety of the application software such as the word processor software has been manufactured and widely used so that users processes and stores a multiplicity of data items by using the thus-processed application softwares.
Then, an example of a portion of the printer engine portion 151 of the conventional laser beam printer will be described with reference to FIG. 2.
Referring to FIG. 2, paper sheets 1 each serving as a recording medium are accommodated in a paper cassette 2. A paper supply cam 3 separates only the uppermost sheet of the paper sheets 1 stacked on a paper cassette 2, and the front portion of the thus-separated paper sheet 1 is, by a conveying means (omitted from illustration), conveyed to a position of paper supply rollers 4 and 4', this paper supply cam 3 being arranged to be intermittently rotated whenever the paper sheet 1 is supplied. A reflection type photosensor 18 detects light reflected by the paper sheet 1 through an aperture 19 formed in the bottom of the paper cassette 2 whereby a fact that there is no paper sheet is detected.
The paper sheet roller 4 and 4' rotates with slightly pressing the paper sheet 1 when the paper sheet 1 is, by the paper supply cam 3, conveyed to a gap by the paper supply cam 3 so that the paper sheets 1 are conveyed. When the paper sheet 1 has been conveyed and the front portion thereof has reached the position corresponding to the resist shutter 5, the conveyance of the paper sheet 1 is stopped by the resist shutter 5 but the paper supply rollers 4 and 4' continue their rotation with slipped with respect to the paper sheet 1 and with generating conveying torque. In this case, when the resist shutter 5 is moved upward by actuating the resist solenoid 6 so as to release the shutting, the paper sheet 1 can be conveyed to the conveying rollers 7 and 7'. The operation of the resist shutter 5 is conducted by arranging a certain timing with respect to the image to be formed due to the imaging of the laser beam 20 on the photosensitive drum 11. The photosensor 21 detects a fact whether or not the paper sheet 1 is present at the position corresponding to the resist shutter 5.
A rotary polygon mirror 52 is arranged to be rotated by a motor 53 for the polygon mirror 52 so that a beam 20 transmitted from a semiconductor laser 51 is introduced into the surface of the photosensitive drum 11 via a reflecting mirror 54. As a result, a recorded image (a Latent Image) is formed on the photosensitive drum 11. A beam detector 55, which is disposed at the position at which the scanning operation with the beam 20 starts, transmits a BD signal when it detects the beam 20, this BD signal serving as a timing signal for starting the writing of the image in the main scanning direction.
Then, the paper sheet 1 is supplied with the conveying torque from the conveying rollers 7 and 7' which supply the torque as an alternative to the paper supplying rollers 4 and 4, so that the paper sheet 1 is conveyed to the photosensitive drum 11 at which the image exposed to the photosensitive drum 11 is transferred to the paper sheet 1 by cooperation of an image cleaner 12, a charger 13, a developer 14, and a transferal charger 15. The paper sheet 1 to which the image has been transferred is then fixed by fixing rollers 8 and 8', and is discharged into a stacker by paper discharge rollers 9 and 9'.
Referring to this drawing, a guide 30 restricts the direction of the conveyance of the paper sheets 1. In addition to the paper feeding from the paper cassette 2, manual feeding of each paper sheet 1 through the paper feeder 16 is able to be conducted. The paper sheet 1 which has been manually fed to the gap from the manual supply roller 17 on the paper feeder 16 is pressed with a light load by this manual supply roller 17. As a result, similarly to the conveyance of the paper sheet 1 conducted by the paper supply rollers 4 and 4, the paper sheet 1 which has been manually fed is conveyed by the manual supply roller 17 to the position at which the front portion thereof reaches the position corresponding to the resist shutter 5. At this position, the paper supplying rollers 4 and 4' are rotated with being slipped. The ensuing conveyance sequence is arranged to be the same as that of the case in which the paper sheet 1 is fed from the paper cassette 2.
The fixing roller 8 includes a fixing heater 24 so that the image recorded on the paper sheet 1 can be thermally fixed by controlling the surface temperature of the fixing roller 8 at a predetermined temperature on the basis of the temperature detected by a thermistor which can be brought into contact with the surface of the fixing roller 8 with being slipped. Reference numeral 22 represents a photosensor capable of detecting whether or not the paper sheet 1 is present at a position corresponding to the fixing rollers 8 and 8'.
The printers such as that shown in FIG. 1 are not used solely, but are each arranged to be connected to the controller by using an interface cable so as to receive a command of printing and an image signal from the controller for the purpose of performing the printing sequence. The structure of the interface cable of the type described above and the signal to be transmitted/received through the interface cable will be described briefly.
FIG. 3 is a view which illustrates various interface-signals to be transmitted/received between a usual printer and a controller of a conventional type. These interface signals will be described respectively.
PPRDY SIGNAL
PPRDY signal is a signal which is capable of notifying a fact that the power for the printer has been turned on so that the printer is able to be operated.
CPRDY SIGNAL
CPRDY signal is a signal which is capable of notifying the printer of a fact that the power for the controller has been turned on.
RDY SIGNAL
RDY signal is a signal which is capable of notifying the controller of a fact that the printer is able to start its action or to continue the same whenever PRNT signal, to be described later, is received from the controller. When the printing action cannot be performed due to, for example, the paper sheet in the paper cassette 2 has been used up, a state FALSE is realized.
PRINT Signal
PRINT signal is a signal for commanding the printer to start printing action, or for commanding the same to continue its printing action if this printer is performing the printing action.
VSREQ Signal
VSREQ signal is a signal which is capable of representing a fact that the printer has prepared for receiving VSYNC signal when both the RDY signal and PRNT signal are in state TRUE, the VSYNC signal being to be described later.
VSYNC Signal
VSYNC signal is a vertical (in the sub-scanning direction) synchronizing signal for the image to be printed, this signal being transmitted from the controller to the printer for the purpose of making the printer synchronize the image on the drum with the paper sheet.
BD Signal
BD signal is a horizontal (in the main scanning direction) synchronizing signal which is capable of representing a fact that the laser beam is positioned at a position at which the laser beam starts its main scanning action.
VDO Signal
VDO signal is a signal representing the image to be printed, this signal being arranged to be transmitted from the controller. Thus, the printer transmits TRUE of this signal as a black component of the image to be transmitted, while the same transmits FALSE as a white component.
SC Signal
SC signal is a bi-directional serial 8-bit signal capable of transmitting both COMMAND and STATUS to be described later. The COMMAND serves as a command signal to be transmitted from the controller to the printer, while STATUS serves as a signal for notifying the state and to be transmitted from the printer to the controller. Both the controller and the printer employ SCLK signal to be described later as a synchronizing signal when this signal is transmitted/received. Since this signal is in the form of a bi-directional signal, SBSY and CBSY signals need to be employed for the purpose of controlling the transmission and receipt of the signals. The COMMAND is a serial signal formed by 8 bits and it comprises the following control commands:
a command of realizing a paper supply mode for bringing the printer into a energy saving state in which only the fixing heater is turned off, that is, a so-called paper-supply mode is realized;
a command of cancelling the paper-supply mode in which the paper supply mode is cancelled and the fixing heater is turned on;
a command of supplying the paper sheet in the paper supply cassette; and
a command of manually supplying the paper sheet.
On the other hand, the STATUS is a 8-bit serial signal for notifying the states of the printer such as a state in which the temperature of the fixer has not as yet been raised to a level at which the printing can be performed and therefore the printer is in an waiting mode, a state in which a paper jam has occurred, or a state in which the paper cassette has exhausted the paper sheet.
SCLK Signal
SCLK signal is a synchronizing pulse signal for making the printer fetch the COMMAND, or for making the controller fetch the STATUS.
SBSY Signal
SBSY signal is used to occupy an SC signal line and an SCLK signal line prior to the transmission of the STATUS conducted by the printer.
CBSY Signal
CBSY signal is used to occupy the SC signal line and the SCLK signal line prior to the transmission of the COMMAND conducted by the controller.
GNRST Signal
GNRST signal is a reset signal for making the controller initialize the state of the printer.
Then, the relative action between the printer portion and the controller portion will be described with reference to a system structural view which illustrates the connection and the structure of the printer and the controller.
It is provided that a power switch for the printer is switched on and a power switch for the control is also switched on. In this case, the printer initializes the internal states of the printer and transmits the PPRDY signal to the controller. On the other hand, the controller initializes the internal state of the controller and transmits the CPRDY signal to the printer. The printer then transmits the RDY signal to the controller, this RDY signal representing a fact that the fixing heater 24 accommodated in the fixing rollers 8 and 8' has been actuated and the temperature of the surfaces of the fixing rollers 8 and 8' have been raised to a level at which the fixing can be conducted.
The controller transmits, after it has received the RDY signal, the PRNT signal to the printer if necessary for conducting the printing. When the printer has received this PRNT signal, the photosensitive drum 11 thereof is rotated and the potentials of this photosensitive drum is initialized equally. Simultaneously with this, the paper supply cam 3 is moved in the cassette paper supply mode so that the paper sheet 1 is conveyed until the front portion thereof reaches the position for to the resist shutter 5. In the manual feed mode, the paper sheet which has been manually fed from the paper feeder 16 is, by the manual supply roller 17, to the position for the resist shutter 15. When the printer is enabled to perform the printing as a result of receipt of the VDO signal, the VSTEQ signal is transmitted to the controller.
The controller transmits the VSYNC signal to the printer after it has received the VSREQ signal. When the printer receives this VSYNC signal, it actuates the resister solenoid 6 in synchronization with the thus-received VSYNC signal so that the resist shutter 5 is released. As a result, the paper sheet is conveyed to the photosensitive drum 11. The controller makes the BD signal transmitted from the printer a horizontal synchronizing signal after it has transmitted the VSYNC signal and successively transmits the image signal VDO to be recorded to the printer, this transmission of the image signal VDO being conducted in synchronization with the horizontal synchronizing signal. The image signal VDO is transmitted in synchronization with the synchronizing clock signal VCLK.
The printer forms a latent image on the photosensitive drum 11 by flashing the laser beam in response to the image signal (VDO signal). The thus-formed latent image is developed by the developer by adhering toner. The thus-developed image is transferred on to the paper sheet by the transferral charger 15, and is fixed by the fixing rollers 8 and 8' before being discharged.
When the paper feed mode of the printer is then switched from the cassette feed mode to the manual feed mode, the controller synchronizes the 8-bit serial codes which correspond to the paper feed modes with the SCLK pulse signal so as to transmit the thus-synchronized 8-bit serial codes to the printer.
In a case where the printer has received the cassette feed mode code, the cassette feed mode is realized in which the manual feed roller 17 is not operated, but the paper supply cam 3 is operated so that the paper is supplied from the paper cassette 2. On the contrary, in a case where the printer has received the manual feed code, the mode is changed to the manual feed mode in which the paper supply cam 3 is not operated, but manual feeding by operating the manual feed roller 17 can be enabled.
When the power for the printer is switched on for the first time, the cassette feed mode is realized as its initial mode.
The signal GNRST is capable of initializing the printer due to the command given by the controller. When this signal from the controller is received, printer stops all of its job and resets it so that the state is reset to the state immediately after the power is switched on. This signal is, for example, used for the purpose of unifying the states of the printers in a case where a plurality of printers are connected to the controller.
FIG. 4 is a view which illustrates a signal processing circuit for use in a conventional laser beam printer. The signal processing circuit acts to input, in synchronization with the clock VCLK for transmitting the image signal, the 1-bit binary signal VDO serving as a black and white image signal and 8-bit parallel multi-level signals VD00 to VD07 serving as half-tone image signals, and this circuit also acts to transmit the thus-input multi-level signals VD00 to VD07 to a dither processing circuit 38. The logical sum of the multi-level signal which has been half-tone processed by the dither processing circuit 38 and the binary signal is calculated in an OR circuit 39, and a synthesized signal is transmitted from the thus-calculated logical sum. The thus-transmitted synthesized signal is input to a laser driver 40 so that a laser 51 is actuated and therefore the printed image is formed on the photosensitive body.
Referring to FIG. 4, reference numeral 37 represents the above-described SC signal so as to be used in a bi-directional serial communication with the controller 152 via a connector 32. Thus, the COMMAND is transmitted from the controller 152, while the STATUS is transmitted from the printer engine 151. Reference numeral 34 represents a print board in which the above-described dither processing circuit 34, the OR circuit, and the like, this print board 4b being connected to the sole connector 32 via a cable 33. The connector 32 is connected to the controller 152 from which the binary signal VDO, multi-level VD00 to VD07, and the clock VCLK are transmitted.
The dither processing method which is conducted by the processing circuit 38 is widely used when half-tone images are expressed with the printers of the type described above. As shown in FIG. 5, this method comprises the steps of:
forming an 8.times.8 matrix;
making comparisons between a predetermined threshold and the image information VD00 to VD07 for each of the pixels; and
printing/non-printing is determined on the basis of the results of the comparisons made as described above.
If an image which uniformly displays its density level of "30" is processed by the matrix shown in FIG. 5, a print formed as shown in FIG. 6 is realized. When all is said, the dither processing method is a half-tone production method by density of each element.
There is a density pattern method available as a similar method to the above-described dither method. The difference of this method from the dither method lies in that the threshold and the image information is not made comparison for each of the pixels, the density pattern method comprising the steps of: forming 8.times.8 density matrices; collectively determining the correspondence between the matrices and the density levels; and printing specific patterns. Since the density pattern method is well known, the description about it is omitted here.
Another method is available for the purpose of expressing the half-tone, that is, a pulse width modulation method. A usual example of the circuit for use in this method is shown in FIG. 7. According to the conventional example shown in FIG. 4, the pulse width modulation processing is arranged to be conducted in the dither processing circuit 38. Referring to FIG. 7, the modulation circuit comprises a pattern signal generating circuit 401, a D/A converter 400, and an AND circuit 402. In the AND circuit 402, a digital image signal which has been dither processed is D/A-converted so as to be subjected to a comparison made with the pattern signal. As a result, the printing is, as shown in FIG. 8, conducted if the image signal is larger than the pattern signal.
FIG. 11 is a detailed block diagram which illustrates a half-tone processing portion of the above-described printer, in particular, a pulse width modulation processing portion is shown. The tone processing portion shown in FIG. 11, for simple description, is shown in the form of a processing portion in which 4-bit data is processed. Referring to FIG. 11, reference numeral 511 represents a .gamma. correction ROM to which image data items VIDEO 0 to VIDEO 7 of 4-bit density (16 gradations) are input from outside and capable of transmitting 4-bit image data which has been 65 -corrected. FIG. 12 is a view which Illustrates a conventional example of the .gamma. correction characteristic. Reference numeral 512 represents a latch capable of latching the .gamma.-corrected image data by using the image clock signal VCLK. Reference numeral 515 represents a counter clock generator capable of generating the counter clock signal SCLK whose frequency is 16 times that of the image clock signal VCLK. Reference numeral 514 represents a counter capable of counting the counter clock signal SCLK. That is, since an input pixel displays 16 gradations, 16 counts are made between the image clock signals VCLK. Reference numeral 513 represents a digital comparator arranged to make a comparison between the output from the latch 512 and the same from counter 514 so as to pulse-width modulate the image data which has been .gamma.-corrected whereby a laser driver (omitted from illustration) is operated with this pulse-width modulation signal.
As described above, the structure and the operation of the conventional recording apparatus such as the laser beam printer, that is, the apparatus for transmitting the image data are arranged. The conventional image data output apparatus such as the laser beam printer encounters the following problems:
(a) When the image formed by using the conventional application program which has been developed for the density of 300 dpi is transmitted to the printer for the density of 600 dpi, high quality recorded image cannot necessarily be obtained. PA0 (b) Since some controllers are arranged to correspond to only binary signals, the function capable of processing multi-level data cannot act if the function of this type is provided for the printer. PA0 (c) If the number of the gradations of the image data transmitted from an external equipment such as a host computer or the like is not the same as that of the printer, any effective action cannot be conducted. PA0 (d) If the .gamma. correction is conducted on the printer side, the number of the gradations can be substantially decreased. PA0 (e) Fogs in white portions and lack in black portions cannot be prevented from generation in the reproduced images, such defects being particularly apparent when the density of the input image signal is converted into a pulse signal which operates the laser according to the above-described pulse width modulation method. PA0 (f) It is impossible to properly control both the resolution and the gradation. PA0 (g) It is impossible to freely determine the angle of the screen.
The problem in the item (a) will be specifically described.
The printer engine portions have been intended to realize a high print density for the purpose of obtaining a higher quality. As a result, printer engines displaying the print density of 600 dpi or higher have been disclosed. The printer controller connected to the thus-disclosed high density printer engine (600 dpi) conventionally include the data memory whose capacity corresponds to this print density (600 dpi). For example, a 600 dpi-printer engine includes a memory of 4 times 300 dpi. The exclusive application software which has been developed for this 600 dpi-printer can be used without any problem. However, a multiplicity of conventional application softwares for the 300 dpi printers cannot be used intact in the 600 dpi high density printers.
The reason for this will be described with reference to FIG. 9.
FIG. 9 is a view which illustrates the structure of dots forming a character "a" to be printed at a print density of 300 dpi. If this character is printed at a print density of 600 dpi with the structure of the dots employed intact, the size Of the character becomes the half in length and width respectively. The reason for this lies in that that the period of the pixel clock when the print density is 600 dpi is the half of that when the same is 300 dpi.
Therefore, the interpolation of data needs to be conducted. In order to interpolate data, a method is available which comprises the steps of: simply doubling the length of the structure of dots in length and width; the structure of dots for the 300 dpi is applied to the 600 dpi. When the structure of dots is converted according to this method, the size of the character can be maintained. However, the rough contour of the character generated at the print density of 300 dpi is intact maintained to the printing conducted at the print density of 600 dpi. Therefor, the thus-printed character cannot exhibit the significant quality which suits the performance of the 600 dpi printer engine.
When, for example, when a multi-level signal is processed by the controller, the cost of the memory of the controller becomes excessive and time taken to complete the processing of the pixel signal is lengthened. Therefore, the number of bits and the resolution of the multi-level signals which can be processed by the controller are inevitably restricted. In other words, due to the restriction in the number of bits and the resolution of the multi-level signal caused from the restriction of the controller side, the multi-level signals cannot be processed and transmitted in a manner suitable for the performance of the printer which exhibits both high grade gradation and improved resolution.
Then, the problem shown in item (b) will be described. When the printer engine portion includes, as shown in FIG. 4, a connector 32 to which both binary signals and multi-level signals can be input, this connector 32 needs to be a connector of a type having a multiplicity of pins although the multi-level signal line is not necessary in a case where the controller to be connected to the printer engine portion comprises a cheap type which is able to process only binary signals. Furthermore, a multi-level signal processing circuit 38 performs a roll to raising the total cost of the printer engine portion. In the other words, it is preferable that the multi-level signal system and the binary signal processing system are arranged to be in the form of a module.
Then, the problems shown in item (c) will be described.
When a half-tone image is output by a printer such as a laser beam printer in which the conventional electrophotographing technology shown in FIG. 1 and so on, a proper binary processing such as a half tone dot processing or the dither processing is conducted by the host computer 154 or the like prior to output of the half tone image data to the printer. According to this method, since the binary signals are used, the data transmission to the printer and data compression can be readily conducted. However, a problem arises in that information about the direction in the depth of the density is lost, causing the resolution of the image of a multi-level image having the number of gradations to deteriorate in particular. Furthermore, due to the difference in the diameter or the density of the dots to be recorded by the printer, the correspondence between the dither pattern and the density of the formed image tends to be varied. That is, if the same dither patterns are input, a low density image is formed in a certain printer since the dots distributed to the relatively white portion are lost, while a high density image is formed in another printer since the dots distributed to the portion adjacent to the relatively black portion are crushed. As a result, it is difficult to form the same images by using a plurality of printers of the different types on the basis of the image data which has been subjected to the same image processings.
Another type of a method has been disclosed which comprises the steps of:
transmitting image data to which information about the direction of the depth for each dot to the printer; and
performing the multi-level output by the printer on the basis of the thus-transmitted information or performing the half-tone output by adding a proper binary processing such as dither processing or the like. According to this method, fine half-tone adjustment can be conducted by the printer and the compatibility can be achieved for the host computer and the printer, respectively.
However, when the multi-level image data of the type described above is transmitted for the purpose of being recorded, the number of the output gradations from the host computer and the number of the gradations processed by the printer cannot always agreed with each other. For example, in a case where the number of the gradations transmitted from the host computer is 6 bits and the number of the gradations input to the printer is 8 bits, the two components above cannot, intact, be connected to each other.
That is, any effective counter measure has not as yet been able to be taken against the case in which the number of the gradations of the image data transmitted from an external device such as the host computer and the number of the gradations of the same processed in the printer do not agree with each other.
Then, the problem shown in item (d) will be described. As can be clearly seen from FIG. 12, although there are 16 gradations from 0 to F in the conventional .gamma. correction, the number the gradations to be output becomes substantially 10. The reason for this lies in that the gradient of the .gamma. correction curve becomes gradual in the input gradation range of "3" to "C". As a result, the number of the gradations of the image data substantially decreases after the .gamma. correction. The same fact is involved in a case where the input image data becomes 8 bits (256 gradations).
The problem shown in item (e) will be described.
In a case where the input image signal VDO is a signal read from an image scanner, the amplitude of the analog video signal VA is, as shown in FIGS. 11 and 13 smaller than that of a chopping wave SAW. It leads to a fact that a noise component such as portions a and b generates also in the portion in which complete white or complete black image is intended to be formed by the printing. The component such as the above-described component a generates a fog, while the component such as the component b generates lack in black portion. Both of these components leads to deterioration in the quality of the image. If the image clock and the chopper wave arranged to have the same period, the above-described problem can be overcome. However, the range in which both dark and light tone can be expressed can be inevitably reduced due to the characteristics of the printer, causing the tone reproduction to deteriorate. Therefore, the structure described above is not preferably used for expressing the half-tone images.
Then, the problem in item (f) will be described.
In the above-described dither method, each of the density degrees is expressed with the size of the predetermined dither matrix. Therefore, when the lowest density (the lightest tone) is expressed in, for example, a usual 8.times.8 matrix, a tone of 1/64 is, as shown in FIG. 14, the theoretical lowest density. Recently, in the printers designed on the basis of the electrophotography system, the major portion of them has includes a light-exposure type printing portion. In the printer of the type described above, a pixel for printing is designed in the form of a circle or an ellipse as an alternative to a square. Furthermore, as shown in FIG. 15, each of the pixels are arranged to be printed with a relatively larger size than a normal pixel since the complete black printing needs to be conducted and the portion in which the non-uniformity caused from the scanning pitch can occur needs to be printed in complete black.
As a result, when the lowest density is expressed, the tone whose density is higher than the above-described theoretical lowest density 1/64 tone becomes the lowest density unit. This problem is also involved in the inject type printers and the like.
Furthermore, when the light tone whose density is at the next level to 1/64 is expressed, 2/64 tone which is the tone in this region can be rapidly changed is needs to be used.
As described above, when the area to be printed is increased by one pixel in the dither matrix, the change in the medium tone can be reduced since, for example, tone 33/64 is the next tone to tone 32/64. However, the above-described light tone whose density is approximated to 1/64 display a rapid visible change. In particular, the tone of the human body or the like which is intended to be expressed can be disordered in these intermediate tone regions, causing a so-called "false contour" to be generated.
In order to precisely express the light tone of the type described above, it might be considered to employ a method arranged such that the area gradation is raised by enlarging the size o the dither matrix. However, although a smooth tone change can be generated in the relatively low density regions, the roughness becomes excessive in the dither matrix, causing a rough image which an insufficient quality to be obtained.
Furthermore, since the actual printing is so conducted as to be larger than one pixel, the portion of the solid black portion from which one pixel is removed can be substantially crushed in a case of a darker-tone regions, for example, in the tone of 63/64. As a result, in the dark tone regions, the difference in the tone between 63/64 and 64/64 becomes little. Therefore, the number of gradations can be substantially decreased.
In an electrophotography type printers, the background exposure type printers arranged such that the non-printed portion is exposed to light encounters the similar problem above, as well as the image exposure type printers. That is, in the printers of this system, a solid black print is formed when the entire surface is exposed to light. In this case, since the portion to be exposed is formed in a shape other than a square and due to the involved scanning non-uniformity, a dot whose size is smaller than a true pixel is used for printing when a pixel is printed (no exposure) in a manner in the contrary to the image exposure system. On the other hand, when one pixel is removed to form a white portion, a dot whose size is larger than the true pixel is used. Therefore, a considerable improvement can be obtained in the light tone regions with respect to the image exposure system. However, the false contour cannot be prevented.
The adverse influence caused from the shape of the spot formed by the laser beam made upon the resolution and the tone reproduction is also involved in the above-described density pattern method not only in the dither method.
Then, an adverse influence of the shape of the spot made upon the gradation expression in the above-described pulse width modulation system will be described. As described above, a multiplicity of gradations can be obtained in a small area according to the pulse width modulation system. For example, when a comparison is made between a pattern signal which forms a cycle with the signals for three pixels and image data, the data expression for the quantity of data about the image information can be conducted simply by signals for three pixels. Therefore, infinite gradation expressions exhibiting high resolution can be obtained.
However, the minimum print width is involved in the electrophotography system due to the particle size of toner, developing characteristics, and laser characteristics. That is, referring to FIG. 16, width a1 is the minimum level with which an actual image can be formed. Therefore, the lightest tone involves a restriction so that the expression is restricted to the limited density expressions as shown in FIG. 17.
Then, the problem shown in item (g) will be described.
When a half-tone image is intended to be obtained in the form of a hard copy, the above-described area gradation method arranged on the basis of the dither method is widely used. This method is arranged such that a matrix 8.times.8 is formed with the pixel density of the printer arranged to be 300 dpi whereby a desired gradation is expressed on the basis of an assignment of the pixel to be printed on the basis of the image data. In this case, the resolution (to be called "the number of lines") is arranged to be 37.5 lines for each inch when the half tone is expressed and thereby the number of gradations is made to be 64. The above-described number of lines and the number of gradations are not limited to the above description. For example, a variety of arrangements such as a matrix of 4.times.4 pixels, 75 lines, and 16 gradation can be employed to meet the requirement by the operator.
The expression of the half tone needs a screen angle to be determined as its factor in addition to the number of lines and gradations. Since the tone of the image is varied due to the position at which the half tone pattern grows and the way of the growth, a required image is formed by varying these factors by determining the screen angle.
FIG. 18 is a schematic view which illustrates a specific example of the half tone pattern when the screen angle is varied. FIG. 18A is a view which illustrates the half tone pattern is allowed to grow diagonally with the screen angle made to be 45.degree., while FIG. 18B is a view which illustrates the case in which the half tone pattern is allowed to grow longitudinally with the screen angle made to be 90.degree.. FIG. 18 C is a view which illustrates a case in which the half tone pattern is allowed to grow laterally with the screen angle made to be 0.degree..
The function of varying the screen angle is widely used and is serving as a critical function in the DTP field.
However, in the above-described conventional technology, the screen angle assigned is involved to be limited by the pixel density of the printer, causing restricted printer angles to be obtained.
For example, when a gradation expression is conducted with 6 pixels.times.6 pixels an 12 pixels of 6.times.6=36 pixels are printed, the printed image becomes as shown in FIG. 19A when the screen angle is 90.degree., while, the printed image becomes as shown in FIG. 19B when the screen angle is 45.degree.. When the screen angle is 0.degree., the printed image becomes as shown in FIG. 19C. As for the intermediate expressions, a pattern shown in FIG. 19D is obtained when the screen angle is 63.degree., while a pattern shown in FIG. 19E is obtained when the screen angle is 27.degree..
As described above, a problem arises in that when the gradation is expressed with 6 pixels.times.6 pixels, the screen angle variation is limited to the above-described five variation. For example, when a screen angle of 85.degree. is intended to be obtained, the expression is involved to be conducted at the angle of 90.degree..
On the other hand, when the half tone is expressed by the above-described pulse width modulation method, there is determined only few types of the pattern signal such as SAW shown in FIG. 13. Therefore, a problem arises in that the screen angle cannot varied freely.
That is, the screen angle cannot be freely and finely determined.
The above-described problems shown in items (a) to (g) is made intensive as follows:
The image output system for laser beam printer and the like comprises: a host computer or a controller such as the personal computer; a printer including a development portion and a fixing portion and capable of transmitting visible images; and a printer engine capable of establishing an interface between the above two components and performing the image formation. Since printers exhibiting a high resolution of 600 dpi has been developed recently, the engine portion needs to harmonize with the conventional host computer side including the application softwares and with the printer portion exhibiting the high resolution. Furthermore, when the host computer side becomes capable of corresponding to the high resolution image formation, the printer engine needs to have a function to maintain the high resolution output performance of the printer.